R help - May 2009 - a general way to select a subset of matrix rows?

Dear fellow R users,
I can't figure out how to do a simple thing properly: apply an operation 
to matrix columns on a selected subset of rows. Things go wrong when 
only one row is being selected. I am sure there's a way to do this 
properly.

  Here's an example:
# define a 3-by-4 matrix x
 > x <- matrix(runif(12),ncol=4)
 > str(x)
 num [1:3, 1:4] 0.568 0.217 0.309 0.859 0.651 ...

# calculate column means for selected rows
 > rows <- c(1,2)
 > apply(x[rows,],2,mean)
[1] 0.3923531 0.7552746 0.3661532 0.1069531
# now the same thing, but the "rows" vector is actually just one row
 > rows <- c(2)
 > apply(x[rows,],2,mean)
Error in apply(x[rows, ], 2, mean) : dim(X) must have a positive length

The problem is that while x[rows,] in the first case returned a matrix, 
in the second case, when only one row was selected, it returned a vector 
(and the apply obviously failed).  Is there a general way to subset a 
matrix so it still returns a matrix even if it's one row?
Unfortunately doing as.matrix(x[rows,]) doesn't work either, as it 
returns a transposed matrix in the case of a single row.

Is there a way to do this properly without writing out hideous if 
statements accounting for single row exception?

thanks,
-peter.

Yes,

use the drop argument;

apply(x[rows,,drop=F],2,mean)


On Sat, May 9, 2009 at 2:33 PM, Peter Kharchenko <
peter.kharchenko@post.harvard.edu> wrote:
> Dear fellow R users,
> I can't figure out how to do a simple thing properly: apply an
operation to
> matrix columns on a selected subset of rows. Things go wrong when only one
> row is being selected. I am sure there's a way to do this properly.
>
>  Here's an example:
> # define a 3-by-4 matrix x
> > x <- matrix(runif(12),ncol=4)
> > str(x)
> num [1:3, 1:4] 0.568 0.217 0.309 0.859 0.651 ...
>
> # calculate column means for selected rows
> > rows <- c(1,2)
> > apply(x[rows,],2,mean)
> [1] 0.3923531 0.7552746 0.3661532 0.1069531
> # now the same thing, but the "rows" vector is actually just one
row
> > rows <- c(2)
> > apply(x[rows,],2,mean)
> Error in apply(x[rows, ], 2, mean) : dim(X) must have a positive length
>
> The problem is that while x[rows,] in the first case returned a matrix, in
> the second case, when only one row was selected, it returned a vector (and
> the apply obviously failed).  Is there a general way to subset a matrix so
> it still returns a matrix even if it's one row?
> Unfortunately doing as.matrix(x[rows,]) doesn't work either, as it
returns
> a transposed matrix in the case of a single row.
>
> Is there a way to do this properly without writing out hideous if
> statements accounting for single row exception?
>
> thanks,
> -peter.
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Henrique Dallazuanna
Curitiba-Paraná-Brasil
25° 25' 40" S 49° 16' 22" O

	[[alternative HTML version deleted]]

One simple adjustment is the following:

apply(matrix(x[rows,],nr=length(rows)),2,mean)


Quoting Peter Kharchenko <peter.kharchenko at post.harvard.edu>:
> Dear fellow R users,
> I can't figure out how to do a simple thing properly: apply an
> operation to matrix columns on a selected subset of rows. Things go
> wrong when only one row is being selected. I am sure there's a way to
> do this properly.
>
>  Here's an example:
> # define a 3-by-4 matrix x
>> x <- matrix(runif(12),ncol=4)
>> str(x)
> num [1:3, 1:4] 0.568 0.217 0.309 0.859 0.651 ...
>
> # calculate column means for selected rows
>> rows <- c(1,2)
>> apply(x[rows,],2,mean)
> [1] 0.3923531 0.7552746 0.3661532 0.1069531
> # now the same thing, but the "rows" vector is actually just one
row
>> rows <- c(2)
>> apply(x[rows,],2,mean)
> Error in apply(x[rows, ], 2, mean) : dim(X) must have a positive length
>
> The problem is that while x[rows,] in the first case returned a matrix,
> in the second case, when only one row was selected, it returned a
> vector (and the apply obviously failed).  Is there a general way to
> subset a matrix so it still returns a matrix even if it's one row?
> Unfortunately doing as.matrix(x[rows,]) doesn't work either, as it
> returns a transposed matrix in the case of a single row.
>
> Is there a way to do this properly without writing out hideous if
> statements accounting for single row exception?
>
> thanks,
> -peter.
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.